Lithography, DNA used to build structures that could lead to new metamaterials | Research and technology | January 2018

EVANSTON, Ill., January 19, 2018 – Researchers have developed a technique for creating optical structures that could lead to new kinds of materials and optical devices. His technique combines a traditional manufacturing method, top-down lithography, with a new method, programmable self-assembly driven by DNA. They used the technique to build optically active super-networks of discrete nanoparticle architectures on a gold surface over large areas. These super-grids, which would be difficult to construct using typical assembly methods, could provide a platform to systematically study and control the light-matter interactions in nanoparticle-based optical materials.

The Northwestern University team took individual colloidal plasmonic nanoparticles of different shapes and sizes, and arranged them in two and three dimensions to form the super networks. Using lithography methods, the researchers drilled small holes – just a nanoparticle wide open – into a polymer resistor, creating "landing pads" for nanoparticle components modified with DNA strands. The configuration of the structures was controlled through DNA molecules containing blocked nucleic acids and confined environments ("landing pads") provided by the pores of the polymer.

Researchers at Northwestern University have developed a new method to accommodate nanoparticles of different sizes and shapes in two and three dimensions, resulting in optically active superlattices. . Courtesy of Northwestern University. The nanoscopic "landing pads" were modified with a DNA sequence and the gold nanoparticles were modified with complementary DNA. The gold nanoparticles modified with DNA were placed in a pre-animated template made of DNA. Piles of structures were built by introducing a second and then a third particle modified by DNA with DNA that complemented the subsequent layers.

By alternating nanoparticles with complementary DNA, the researchers were able to build nanoparticle piles with tremendous positional control over a large area. The particles can have different sizes and shapes, such as spheres, cubes or discs.

The resulting architectures had tunable arrangements and independently controllable distances in nanometric and micrometric length scales.

"This approach can be used to build periodic lattices from optically active particles, such as gold, silver and any other material that can be modified with DNA, with extraordinary precision at the nanoscale," said Professor Chad Mirkin.

Researchers used numerical simulations and optical spectroscopy techniques to identify which wavelengths of visible light were absorbed by the different super-domains. They reported that the structures could exhibit almost any color in the entire visible spectrum.

In addition, it was shown that the materials are sensitive to stimuli: that is, the strands of DNA that hold them together change length when exposed to new environments, such as ethanol solutions that vary in concentration. The change in DNA length, the researchers discovered, caused a color change from black to red to green, providing an extreme adjustability of the optical properties.

"Fine tuning the optical properties of metamaterials is a significant challenge, and our study achieves one of the highest tuning ranges achieved to date in optical metamaterials," said Professor Koray Aydin.

The team believes that the technique could be used to construct metamaterials for a range of applications that includes sensors for medical and environmental uses.

"Our novel metamaterial platform, enabled by the precise and extreme control of the shape, size and spacing of gold nanoparticles, is very promising for the latest generation of optical meta-materials and metasurfaces," said Aydin.

"Architecture is everything when it comes to designing new materials, and now we have a new way to precisely control particle architectures in large areas," said Mirkin. "Chemists and physicists will be able to build an almost infinite number of new structures with all kinds of interesting properties, which can not be done by any known technique."

The research was published in Science (doi: 10.1126 / science.aaq0591).